Power plants for regenerative energy are typically connected via a main converter to a power grid. Power plants for regenerative energy may comprise wind turbine plants (WTP) or tidal turbine plant (TTP), using a generator for generating electrical energy. The main converter comprises means for adapting the power provided from the power plant, so that it can be introduced into the power grid. Hence, the main converter can perform adaptation of voltage levels, frequency and/or phase of the power plant to the power grid.
A typical main converter for connecting power plants for regenerative energy to a power grid comprises a grid breaker for connecting to and disconnecting from the power grid, and at least one converter path comprising a central DC link. Further converter paths can be provided in parallel depending on the design of the main converter, whereby the converter paths may have a similar or identical design. Each of the at least one converter paths comprises a central DC link, a grid side converter and a generator side converter, both connected to the DC link. In some embodiments, the main converter further comprises a main transformer for adapting the voltage level of the provided energy to the voltage level of the power grid. In some embodiments, the main transformer is provided separately from the main converter.
The grid breaker (GRB) can be used to disconnect the main converter together with the transformer from the power grid. When the main transformer is provided, separately from the main converter, only the main converter with the converter paths is disconnected by the GRB. Accordingly, if the main converter should be connected to the grid, the main converter's DC link has to be charged. When the main converter comprises the main transformer, also the main transformer has to be magnetized prior to connecting it to the power grid, since the main transformer is major source for inrush currents upon connection to the power grid. Pre-charging reduces inrush currents, which may harm the main converter, to a minimum. Furthermore, also the impact of the inrush currents to the power grid voltage level is minimized by connecting a pre-charged main converter instead of a uncharged one. Pre-charging may also include optional components of the main converter, e.g. a high frequency grid filter provided at the power grid side of the grid side converter.
State of the art WTPs or TTP may have a topology, in which the main transformer is not disconnected at any time from the grid when the converter is disconnected. The main transformer stays always magnetized. If the main converter then is connected to the grid, the resulting inrush current is smaller, because it originates only from the charging of the main converter's DC link and from the magnetization of further optional components, e.g. a high frequency grid filter. This topology means that the GRB is located at the main transformer's secondary side.
Another established solution is the usage of a pre-charge unit (PCU) for feeding the main converter from the power grid, typically via an auxiliary transformer. This PCU loads the main converter's DC link with nominal DC voltage, as long as the grid side converter magnetizes the main transformer and e.g. an optional high frequency grid filter. The GRB is not closed until the magnetization is finalized, i.e. means voltage amplitude and phase on GRB's primary and secondary side are the same. This way, an inrush current is avoided or at least highly reduced. Typically, the used PCU is dimensioned to deliver the energy that is needed to magnetize the main converter only. This means that the power class of the PCU is much lower than the power class of the WTP or the TTP.
A third established solution is a phase selective switchgear which closes the single phases of the GRB in the moment of each phases voltage amplitude is at its maximum. This type of phase selective switchgear is much more expensive than a usual one.
WO 2013/007288 A2 discloses a method for connecting a main converter, especially for use in a power plant for regenerative energy, having a generator, to a power grid, the main converter comprising a grid breaker provided at a power grid side of the main converter, and at least one converter path, each converter path comprises a DC link, a grid side converter and a generator side converter, both connected to the DC link.
In R. Galindo del Valle et al. “On the Emulation of an Isolated Wind Energy Conversion System: Experimental Results”, ELECTRONICS, ROBOTICS AND AUTOMOTIVE MECHANICS CONFERENCE, 2009. CERMA 09, IEEE, PISCATAWAY, N.J., U.S.A., 22 Sep. 2009, an isolated wind energy conversion system is disclosed, which is composed by a doubly-fed induction generator, a back-to-back converter connected to its rotor and a LC filter.
DE 10 2007 021 930 A1 shows a well-known and generic method for switching-ON of a power semiconductor in a converter plant.
WO 2012/163398 A1 shows a system for distributing electric power to an electrical grid, whereby the system comprises a DC/AC inverter arranged to convert a DC voltage output from an electric power generator to an AC voltage, a transformer arranged to receive said AC voltage, transform said AC voltage and deliver the transformed AC voltage to the grid and a connector arranged to selectively connect and disconnect the transformer from the grid.